In a liquid chromatograph mass spectrometer (LCMS) in which a mass spectrometer is used as the detector for a liquid chromatograph, a mass spectrometer which employs an atmospheric pressure ion source capable of directly ionizing a liquid sample is generally used. In this type of mass spectrometer, samples are ionized under generally atmospheric pressure, while the mass spectrometry of the generated ions is performed with a mass analyzer (such as a quadrupole mass filter) placed in an analysis chamber in which a high-vacuum atmosphere is maintained. To maintain the degree of vacuum within the analysis chamber, one or more intermediate vacuum chambers with the degree of vacuum increased in a stepwise manner are provided between the ionization chamber maintained at atmospheric pressure and the analysis chamber (i.e. the configuration of a differential pumping system is adopted). The neighboring chambers are separated by a partition wall having an ion-passing hole with a small diameter, through which ions are transported.
In order to efficiently transport the ions, an ion transport optical system (which is generally called an “ion lens” or “ion guide”) for focusing the ions, and for accelerating or decelerating ions in some cases, by the effect of an electric field is provided in each intermediate vacuum chamber. A sampling cone, skimmer or similar tapered device provided on the partition wall separating the chambers, with the aforementioned ion-passing hole formed at its apex, can also be regarded as one kind of ion transport optical system, since those devices also have the effect of focusing, accelerating or decelerating ions by the electric field created by an appropriate amount of voltage applied to them. Similarly, the quadrupole mass filter placed in the analysis chamber, and a prefilter provided before the mass filter can also be regarded as one kind of ion transport optical system. Thus, mass spectrometers are provided with a plurality of ion transport optical systems which influence the flight path of the ions by the effects of electric fields.
In an atmospheric pressure ionization mass spectrometer, a certain amount of unwanted particles (such as the neutral particles originating from the solvent or similar substances, or the fine droplets from which the solvent is incompletely vaporized) are inevitably introduced into the intermediate vacuum chambers or analysis chamber in addition to the ions to be analyzed. Such unwanted particles often attach to the previously mentioned ion transport optical systems and accumulate on their surface. If an insulating film is formed by contaminants or foreign substances attached on the surface of an ion transport optical system, the ions impinging on that portion are likely to cause electrification, or the “charge-up” (for example, see Patent Literature 1). The charge-up can also occur due to the ions which come in contact with a structure made of a ceramic, synthetic resin or similar insulating material which are provided to hold a quadrupole mass filter, ion guide or similar system at a fixed position within a space. Allowing too much of a charge-up leads to a disturbance of the electric field formed in the ion-passing space by the voltage applied to the ion transport optical system, which impedes the passage of the ions or prevents the correct focusing or acceleration of the ions, with the consequent decrease in the amount of ions reaching the detector. That is to say, the ion intensity may possibly decrease as the measurement continues.
FIG. 4A is a chromatogram showing the intensity of the ions detected from a standard sample by an LCMS employing a quadrupole mass spectrometer into which the standard sample was repeatedly introduced at predetermined intervals of time. Each peak in the figure is the ion peak originating from the standard sample. Normally, the peak should always occur with the same intensity. However, the obtained result shows that the peak intensity gradually decreases with time, or with the repetition of the measurement. According to an experiment conducted by the present inventor, this decrease in the ion intensity is most likely due to the charge-up of the quadrupole mass filter.